Remove use of indirect obsolete api usage

Summary: `getOutputFile` calls `getPackageApplication` which is obsolete. `getOutputFile` isn't our code, but we can work around this to get rid of the warning.

Reviewed By: sjkirby

Differential Revision: D17989008

fbshipit-source-id: 127eef28467d47ac5f284b00ebfc94dfa199f85c

plugin/src/main/kotlin/com/facebook/testing/screenshot/build/PullScreenshotsTask.kt

@@ -41,7 +41,12 @@ open class PullScreenshotsTask : ScreenshotTask() {
 
   override fun init(variant: TestVariant, extension: ScreenshotsPluginExtension) {
     super.init(variant, extension)
-    apkPath = variant.outputs.find { it is ApkVariantOutput }!!.outputFile
+    val output = variant.outputs.find { it is ApkVariantOutput } as? ApkVariantOutput
+        ?: throw IllegalArgumentException("Can't find APK output")
+    val packageTask = variant.packageApplicationProvider.orNull
+        ?: throw IllegalArgumentException("Can't find package application provider")
+
+    apkPath = File(packageTask.outputDirectory, output.outputFileName)
   }
 
   @TaskAction